Don't recommend specific GCC version in installation documentation

[Roland McGrath]

I think the general recommendation to try the latest stable GCC is fine.
But rather than saying just that, we need to say something like, "At the
time of this release, GCC versions up to @testedgcc{} are known to work.
If newer GCC versions have trouble building a working library and there is
no newer release of @theglibc{} you can use instead, then you may need to
an older GCC version to build @theglibc{}."